There is a vast array of possible reasons for this. Without some more information or a code snippet, this is just a guessing game.
Doing some blind guessing: Check your viewport settings, aspect ratio, resolution

If the monitor does not support the requested resolution, SDL tries to emulate it. To avoid this, try to pick a resolution is directly supported. You can list them using SDL_ListModes: http://sdl.beuc.net/sdl.wiki/SDL_ListModes.

If the monitor does not support the requested resolution, SDL tries to emulate it. To avoid this, try to pick a resolution is directly supported. You can list them using SDL_ListModes: http://sdl.beuc.net/...i/SDL_ListModes.